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Hydrocarbons Technology :: SSP

Crude oil distillation/Topping of crude oil/Crude oil fractionation

High resolution flow-diagram is incorporated at the end of this document.

[A] Atmospheric Distillation Unit (ADU)

Typical crude oil contains: C1 to more than C100 hydrocarbons with boiling point varies -160 to +1000 oC.

50,000 to 1,20,000 barrel/day crude oil processing 35-40 m height and diameter 5-8 m 30-40 trays Sieve /bubble cap/valve trays Pressure : 1-1.5 atm & Temperature varies from Top to Bottom Steam: 4 to 5 kg/barrel of crude oil processed R for light fractions = 2 to 3 R for heavy fractions = 1.5 to 2.5

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Types of distillation observed in ADU:

Continuous distillation/fractionation Flash distillation Steam distillation

The crude oil is heated upto 350 oC and flashing is done at lower part of the ADU. This converts majority part of liquid crude oil into vapour forms. Addition of steam at the bottom of ADU reduces the average boiling point of oil. By combined all these effects (heating upto 350 oC, flashing and steam addition) the reboiler in ADU can be eliminated. If the reboiler is installed in ADU the cracking of crude oil starts in reboiler tubes due to local high temperatures which cause the coke formation as well reduction in the purity of side cuts. Also the circulation of crude oil in reboiler tubes is a challenge.

Over-flash:

It is the portion of total vapour leaving the flash zone boiling above the nearest side draw fractions but never included in that fraction.

This maintains a good pool of liquids & reflux on plates.

2% crude fed to ADU & VDU is maintained as over-flash.

Increase in flash zone pressure increase draw temperature or Pressure in flash zone is reflected throughout the column in the form of plate temperatures.

Increase in steam in flash zone decrease the product plate temperature.

[B] Vaccum Distillation Unit (VDU)

The ADU residue (ADR) is fed to the VDU where vaccum is applied to reduce the average boiling point of ADR in order to avoid the cracking of oil. The distillation is carried out at 350-400 oC under vaccum.

Packed bed tower is used. The low L/D ratio is used for vaccum tower compared to ADU

Pressure : at Top 12-15 mm Hg, at bottom 40-60 mm Hg Steam: 0.3 to 5 kg/barrel of crude oil processed Recycle ratio = 1.5 to 2.5 2% over flash

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Vaccum Distillation

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Product fractions of ADU & VDU

Fraction Avg. B.P, oC Next destination Ultimate productsGases/LPG Up to 35 Sweetener Propane fuel, LPGLight Nephtha 35-85 Hydrotreater Gases, GasolineHeavy Nephtha 85-200 Cat. Reformer Gases, Gasoline,

AromaticsKerosene 170-270 Hydrotreater

Parrafin separation

Jet fuel

Diesel 250-300 Hydrotreater DieselGas oil 300-340 Hydrotreater

& FCC, Hydrocracker

Heating oil, diesel

VGO 300-500 Hydrotreater& FCC, Hydrocracker

Gasoline, lub. Oil, gases, fuel oil,gasoline, jet fuel, diesel, FCC feed

Vaccum residue >615 CokerVisbreakerAsphalt unitHydrtreater

Coke, gas oil, gas oil, residue,desphalted oil, asphaltFCC feed

Typical fractions obtained upon fractionation of various crude oils

APISp. gravity Sulfur wt %

Norway Nigeria USA M.East.38.3, 0.8333, 0.37

35.4,0.8478.0.14

30.1,0.8478,2.0

24.6,0.90,3.9.

Yield, Wt %Light ends 2.3 1.5 1.5 1.1Naptha 30.1 27.7 16.9 15.8Kerosene 9.9 12.5 8.5 7.4Atm. Gas oil 15.1 21.6 14.1 10.6Light VGO 17.6 20.7 18.3 17.2Heavy VGIO 12.7 10.5 14.6 15.0Vaccum residue

12.3 5.5 26.1 32.9

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Arrangement of Distillation Towers or Reflux management

(a)-top tray reflux, (b)-pump back, (c)-pump around reflux.

In Top tray reflux arrangement the reflux is provided from the top the tower only under such arrangement the side product fraction qualities deteriorate. No refinery uses such arrangement.

In pump back reflux arrangement the reflux liquid from higher tray to lower tray is sent. This arrangement is most effective in order to enhance the side cut product quality.

In pump around reflux arrangement the reflux from lower plate is sent to the higher one this causes the un-even mixture of liquids (mixing of high boiling point and low boiling point liquids) at a given plate. This arrangement is not promising for the ADU.

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Gasoline (Petrol)

Finished product, raw fraction is Naphtha Avg. B.P. 35-185 oC Use: automobiles and aviation Used in spark ignition (SI) internal combustion engine

Source in Refinery:

Light and Heavy Naphtha ADU gases: using Alkylation, Polymerization. ADU gas oil & light VGO: using catalytic cracking Light & heavy VGO: using catalytic hydrocracking VDU- residue: using coking, visbreaking.

Composition:More than, 1500 hydrocarbon compounds

Mainly C5 to C10

Iso-paraffins n-paraffins Cyclo paraffins Aromatics Olefins S-compounds upto permissible limit Additives Composition vary with source

Gasoline is reported in terms of “Octane no.”

O.N. 85-87 regular 91-93 premium

O.N. = “% vol. of i-octane (2,2,4-trimethyl pentane) in a mixture of i-octane and n-heptane that gives the same knocking characteristics as the fuel under consideration.”

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Comparison of automobile fuels obtained from crude oil

Gasoline (Petrol) Jet fuel Diesel

Iso-paraffins

C5 to C12

Iso-paraffins+Naphthenes

C11 to C13

n- paraffins

C13 to C17

High Auto-ignition Temp. High Auto-ignition Temp. Low Auto-ignition Temp.

SI Engine SI Engine CI Engine

Smooth running Smooth running Vibration

O.N. : 75-95 O.N. & Smoke Point C.N. : 45-55

& Diesel Index

Avg. B.P. 85-150 oC 170-270 oC 250-300 oC

Other HC:

n-paraffins (least)

Aromatics*

Olefins*

Other HC:

n-paraffins (least)

Aromatics

Olefins

Other HC:

i-paraffins

Aromatics

Olefins

Sources in Refinery:

Light Naphtha, Heavy Naphtha, Thermal & Catalytic cracking, Hydro cracking, Alkylation and Poln

of light H.C.

Kerosene cut

Catalytic cracking

Hydro cracking

Diesel cut

Hydro cracking

Coking

*In Gasoline, even if Hydrocarbons are having good Octane Number (ON) they are removed to minimize tail pipe emission of them. The olefins are minimized to inhibit oligomerization (Gum formation). Gasoline engines are smooth compared to diesel engines.

The auto-ignition temperature of aromatics is relatively higher however up to 15% aromatics are allowed in diesel due to high energy content of aromatics. Addition of aromatics decreases the cetane number. Therefore the desired cetane number for diesel is kept 45-55. Normally diesel is used for heavy duty compared to gasoline.

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For Jet fuels the heavy duty is required which is achieved by adding Naphthenes (saturated aromatics) with intermediate auto-ignition temperature. Diesel is not used as a fuel for air-crafts due to high compression is required for CI engines and also the vibration in CI engines is more.

Compound Auto-ignition Temperature, oC

Boiling Point, oC Flash Point, oC

n-octane 220 125 25

i-octane 417 99 5

n-cetane (C16H34) 201 287 135

Knocking:

Knocking is untimely burning of fuel in S.I. engine results in loss of power & cause damage to engine parts. An unusual sharp metallic audible sound under

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certain operating conditions. This gives direct thud to the engine structure and eventually leads to engine failure.

Pre –Ignition:Some spots of combustion chamber such as body of spark plug, exhaust valve or carbon deposits became over heated under running conditions. These spots act like a spark plug and ignition for fresh charge. Thus totally different flame front is formed. This means combustion initiated before regular spark takes place. This is uncontrolled combustion & erratic in nature.

“Power ↓ because greater compression work needed to compress the products of combustion.”

Control of Knocking

Using high octane fuel Adopting low compression ratio Improving combustion chamber design to ↓ flame level.

Octane number (ON)

It decides the composition of gasoline

n-heptane: low auto- ignition temperature i-octane: high auto-ignition temperature

Desirable composition of gasoline

Ideally, i-octane → but not possible

Maximum branched chain paraffins Minimum n-paraffins. More than C5 n-paraffins must be minimize. Aromatics, good O.N. but due to environment regulations tail pipe

emission to be reduced so minimum aromatics allowed. Cyclo paraffins: have intermediate O.N. Olefins: good O.N. but not preferred due to gum formation.

{Gum formation: olefins are unsaturates → undergo auto-oxidation or oxidation in presence of air. Auto oxidation →progressive chain reaction → hydroperoxide →free radicals → alcohols, Aldehyde, ketones, acids, non-volatile high molecular weight Polymerization material (gummy)}.

Cause, deposit in carburettor, venture and combustion chamber.

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Kerosene and Jet fuel

Kerosene used for house hold lightening in olden days. Also used for pesticide and Linear Alkyel Benzene (LAB) etc.

Jet fuels are obtained in the same avg. boiling point cut as Kerosene. Around 90%, 170-270 oC boiling point cut is converted into the jet fuels. Composition:

i-paraffins + Naphthens → Jet fuel Aromatics upto some extent to ↓ smoke and to increase heat

content Olefins as minimum as possible to avoid gum formation. n-paraffins must be separated as they posses low auto-ignition

temperature.

Jet fuels used in S.I. engine (Turbine based SI Engine).

High heat content is achieved by using appropriate proportion of Naphthenes and i-paraffins.

Naphthenes have high heat content/volume. Paraffins have high heat content/wt.

Sources of Jet Fuel and Kerosene in Refinary:

ADU cut at 170-270 oC. Avg boiling cut. Hydrocracking of gas oil, light VGO, coker gas oil, vaccum residue. A

main product of hydrocracking is gasoline.

Kerosene: same composition as jet fuels are desired however no bar for the n-paraffins. This is due to kerosene is used for illumination purpose only.

Diesel

Sources in Refinery:

Avg. B.P. cut in ADU: 250-320 oC Hydrocracking of gas oil, light VGO, coking etc

Diesel for automobiles need good ignition, quality, ensure easy starting, smooth running and very low sulfur content.

Diesel Engine:

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Compression Ignition (CI) engine

Stages:

Compression of air upto a desired pressure. High compression ↑ temp. up to 500 oC or more. Diesel is injected in the form of fine droplets. Combustion will take place

without spark. Combustion of H.C. followed by continuous burning of incoming fuel

from injector. The sooner ignition takes place after injection of fuel starts, the

smoother and quieter will the engine operate. Ignition Delay: The time interval between injection and onset of

the sufficient pressure to achieve self ignition temp, of H.C.For a given diesel engine, higher cetane no. causes, shorter ignition

delay period →less combustion noise and good combustion quality.

Composition of Diesel fuel:

H.C. with auto ignition temperature in the range of 200 to 400 oC are suitable for the diesel.

More volatile H.C. ≈ higher auto ignition temperature. C13 to C17

n-paraffins and aromatics with naphthenes, olefins, impurities etc

Properties of Diesel:

Cetane no: % volume of n-cetane in a mixture of n-cetane and α-methyl naphthalene, which gives the same ignition delay as the fuel under consideration, when tested in a std. Engine.

Cetane, also called as n-hexadecane (C16H34).

Significance: Indicates diesel quality in terms of combustion. Higher the C.N. lower the ignition delay. Higher the C.N. smooth and efficient performance of diesel engine.

Diesel Index (DI):

DI= (0.018AP, oC + 0.32 )API

DI ↑ AP↑ Aromatic content ↓

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DI = 45 to 55 for all diesel

Low speed → high speed diesel.

Auto-ignition temp. for n-paraffins is low (C13-C17 up to 400 oC)

Auto-ignition temp. for Aromatics is high (i.e. Benzene 651 oC)

Ideally, high D.I. is desirable, but advantage of Aromatics is that it has high heat content than paraffins.

Therefore, optimum 45-55 diesel index are to be used.

Therefore, some ‘Diesel Knock’ has to be tolerated.

Gasoline Blending

The gasoline is obtained at many sources in the refineries as we have discussed during crude oil fractionation. The octane number and impurities content of all these gasoline are different. Some are having very high octane numbers and others are having poor. Therefore the gasoline blending has to be carryout carefully in order to maintain the market specifications, combustion quality and environment norms. The additives are also to be blended during such operation. A typical continuous blending flow diagram is shown here.

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Crude Assay:Importance

Crude oil processing. Processing capability. Crude oil purchase feasibility. Establishment of compatibility against variation in crude composition. Requirement of crude oil purification before A.D.

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Provides possible % quantum of fractions. Information about possible difficulties be set during treatment

operations. Budget estimation for new refinery to be set-up.

[A]. Physical Properties:

1). API gravity:

= sp. Gravity of sample at 15.60C/15.60C.

2). Salt content:

Read to determine corrosiveness of oil.

Conductivity method:

water to oil Res

3). Metals:

By product formation Determined by: AAS, XRF.

4). S-compounds:

XRF ASTM D4294 or DS291 upto 5%. 5% S, combustion Teach. ASTM D5453. Mercaptan sulphur: potentiometric titration (sample+IPA+NaOH)

titration with silver Nitrate.

5). Total acid number(TAN):

Acidity of oil/fractions, ASTM D664

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(Sample in tolune/IPA/water)-titration with KOH. TAN=mg KOH/g of sample (0.05-6) Causes corrosion problems.

6). Aniline point:

“It is defined as the lowest temperature at which aniline is soluble in a specified amount of sample”.

Significance:

Aromatic content of sample Diesel quality Solvency power of H.C.

7). Pour point :

“Defined as the temperature normally 3 degrees above the point a sample no longer moves when inverted”.

Important for transportation of crudes in pipe lines. ASTM D5852, for -360C ASTMD97.

8). Freeze point :

“Temperature at which crystal start to form in H.C, liquids and then disappear when the liquid is heated.”

ASTM D2386.

9). Nitrogen compounds:

Catalyst poisioning

Oxidative combustion ASTM D3228

Or Kjeldahl method: labour intensive involving digestion /distillation and titration.

10). Cloud point :

“Temperature at which a haze appears in a sample which is attributed to the formation of wax crystals.”

Significance: useful to determine tendency of small orifices to plug in cold operating temps.

‘ASTM D2500’.

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11). Smoke point:

“Maximum flame height in a lamp without smoke formation.”

High value represents clean burning fuels. ‘ASTM D1322’

12). Viscosity:

Standard V.M. like redwood V.M is used . Normally ‘ α 1/T ’ and gives ,straight line on semi-log graph.

13). Micro carbon residue:

C-Residue α Asphalt content Condradson carbon ASTM D189.

14). Asphaltic crude oils:

Penetration : to determine consistency of semi-soild oil. Softening point : temp at which hard asphalt reach on arbitrary degree

of softening.

B]. Distillation Characteristics:

1). Reid Vapour Pressure (RVP):

RVP is measurement of volatility of H.C. liquid. Whole crude with high API (less Sp. g.r.) are subjected for RVP

measurement .

2). True Boiling point (TBP) curve:

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TBP curves

Normal Boiling Point monogram (corrections with respect to higher pressure)

(2). ASTM Distillation or Engler Distillation ASTM D86:

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(3). Equilibrium Flash Vaporization (EFV):

Quick flashing without fractionation

Calculation of equilibrium composition when flash distillation is carried out.

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